As a method of joining a workpiece including two members, friction stir welding is known. The friction stir welding is a method which joins a workpiece using friction heat generated on a workpiece surface by rotating a tool in a state where a joint part of the workpiece is pressurized by a shoulder surface of the tool.
Meanwhile, in the related art, inspection of a joined state is performed on the joint part of the friction stir welding by Ultrasonic Testing (UT) after the joining.
In the Ultrasonic Testing, it is possible to inspect internal defects or a joint diameter disclosed in PTL 1, for example. It is possible to prevent a pressure-joint surface region from being misidentified as a diffusion joint region and accurately measure a diffusion joint diameter by using the method of PTL 1.